Currently, magnetisation states in a ferromagnetic component, as e.g. a ferromagnetic MRAM cell, are being manipulated, e.g. switched or changed, and assessed, e.g. read or written, by magnetic fields generated by neighbouring electrical currents, or by applying external magnetic fields.
The technique of magnetic field induced switching by current conductors is widespread and is currently used in a wide series of commercial products. Several types of magnetic field induced switching by current conductors are known. The switching is generally done by a static method, where currents high enough to switch the element are applied and the element switches after waiting long enough. An alternative method for driving magnetisation read-out or for changing the magnetisation state of a ferromagnetic component is making use of ferromagnetic resonance (FMR). Ferromagnetic resonance is an intensively studied phenomenon, which is well known, and its use for the switching and assessment of ferromagnetic components offers several speed and power advantages as compared to regular methods. The mechanism known in the art as ‘precessional switching’ is based on the ferromagnetic resonance properties of the magnetic device and allows magnetisation reversal with less power and at higher frequencies than with other, older switching schemes.
All the above described techniques, however, have several different problems, such as e.g. current lines are needed for both biasing and magnetic assessment, a bit selection scheme has stringent timing requirements, power consumption is relatively high and different metallisation levels are required. Furthermore, reference cells can be necessary for comparing states during read-out, which reduces the effective cell density. Typically one reference bit per data storage bit is used.
Operating at ferromagnetic resonance frequencies leads to difficulties in controllability and integration. Moreover there is the constant need for external magnetic fields to control the magnetic properties, which limits the use of magnetic materials, even at low frequencies, due to field spreading and power consumption. The latter makes it hard to use FMR in several applications.
It is furthermore a known characteristic of magnetic materials that their magnetic state can be altered by the presence of stress and/or strain in a magnetic material. A typical suitable material for stress state alteration is Ni which is described e.g. in Sander D., “The correlation between mechanical stress and magnetic anisotropy in ultrathin films”, Reports on Progress in Physics 62, (1999) p 809. Typically, stress is induced by applying a voltage to a piezo-electric material and the use of stress is only known to be controllable at low frequencies. This limits the use of stress induced switching in e.g. ferromagnetic memory cells.